The present invention relates to digital-to-analog conversion, and more particularly to a method of high-speed digital-to-analog converter (DAC) linearity measurement in the presence of noise, quantization error and other distortions.
Linearity is an important specification in a wide variety of applications using DACs. For speed one clock period per DAC value (output level) may be required. Due to both uncorrelated and correlated (such a pixel clock) noise, distortions and other signal impairments a problem in defining a start and end of a DAC output level may arise. FIG. 1 shows a zoomed one least significant bit (LSB) step staircase “ramp” signal per the Video Electronics Standard Association (VESA) standard output from the DAC on a video graphics card. The ramp signal was acquired by a digital oscilloscope having an 8-bit data acquisition section using a 500 waveform average of a low jitter signal. Note the lack of distinct steps in many areas of the waveform.
In order to comply with existing VESA standards, computer graphics card manufacturers need to automatically measure the resolution, monotonicity, differential linearity (DNL) and integral linearity (INL) of DACs in the presence of noise and other distortions. For speed and convenience the VESA standard specifies that a video line ramp input signal is used for this purpose. Due to the number of samples per video line and the number of output levels of the DAC that need to be measured, these ramps often are limited to one pixel per level at pixel rates of hundreds of mega-pixels per second. At these rates the graphics cards often have jitter and/or other pixel clock errors.
Currently the 8-bit data acquisition of high-bandwidth digital oscilloscopes is used for this measurement, despite the fact that the measurement is used for DACs having a greater resolution than 8 bits, i.e., 10-bit DACs. Therefore in addition to the above-mentioned signal impairments, the-test instrument platform (digital oscilloscope) also introduces quantization error and noise.
In known automatic methods for computer graphics cards the measurement of DAC linearity involves (i) a priori knowledge of the locations (time windows) of the levels to be measured, including measuring “DC levels”—holding the output constant while the voltage measurement takes place, or (ii) for dynamic signals knowledge of clock rates, level positions on the ramp, etc. Related prior automatic methods for professional and television video linearity measurement do not apply because the ramp steps are filtered using reconstruction filters in these band-limited video applications, i.e., all steps on the ramp are removed which removes resolution information. The VESA standard requires measurement of the resolution, which means there is not a priori knowledge of how many steps there are on the required ramp. The video line ramp requirement means that a lower speed approach may not be used.
What is desired is a high-speed method of automatic measurement of resolution, monotonicity, differential linearity and integral linearity for DACs in the presence of random noise, quantization error and other non-linear and linear distortions and interferences.